Pistons are useful for many things in today's world. For example, internal combustion engines that run our cars use pistons. Pistons can also be used for hydraulics, pneumatics, pumps, and other devices of all shapes and sizes. The possibile uses for pistons is endless. However, it is very difficult if not impossible to make a perfectly sealed piston for your everyday projects. In this Instructable I will show the process I used to make perfectly sealed pistons for PVC pipes, and how I used a piston to make a simple airgun.

A video of the airgun can be found here: http://www.youtube.com/watch?v=WGaR2vzxUu8
I had a lot of difficulty getting the video to embed properly, even though I have not before. If anyone has a tip, let me know!
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Webster's Definition of "piston": a sliding piece moved by or moving against fluid pressure which usually consists of a short cylindrical body fitting within a cylindrical chamber or vessel along which it moves back and forth
See source here: http://www.merriam-webster.com/dictionary/piston
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Here's an overview of what will be explained in the following steps:

Step 1: Materials Needed
Step 2: The Build: The Secret to the Perfect Piston
Step 3: The Build: Piston Pictures
Step 4: Simple Airgun Theory of Operation
Step 5: The Build: Simple Airgun
Step 6: Conclusion

Step 1: Needed Materials

Not many materials are needed for this project. The list is as follows:

PVC Pipe - (pic 1) - You can use whatever diameter you would like. In this project, I use a length of 1" Inner Diameter Schedule 40 PVC for the outer part of the piston. For the inner piece that seals against the 1" pipe, I used 1/2" Inner Diameter Schedule 40 PVC. PVC pipe is available at any big box home improvement store.
Foam Rubber Sheet- (pic 2) - I bought this sheet at my local craft store. It is very cheap, and comes in a variety of colors and thicknesses. In this Instructable I am using black "Foamies" brand sheets that are 3mm in thickness.
Hot Glue Gun - (pic 3) - Everyone has to have one
Dremel or Rotary Tool - (pic 4) - I use my Dremel for nearly every project. Optional, but very helpful.
Hacksaw - (not pictured) - I used this to cut the handles for the airgun I used the piston for. Optional.

Step 2: The Build: the Secret to the Perfect Piston

Procedure:

Get out the foam rubber sheet and the PVC pipe. You should either have two pieces of PVC, one for the piston and one for the outer cylinder, or have some other object to become the piston.
Cut a piece of foam rubber equal to the circumfrence of the object to be used as the piston. In my case, I loosely wrapped the foam rubber around the pipe, marked the length with a Sharpie, and cut it there. The strip was adjusted to precise length afterward. The ends of the strip were cut at an angle so as to overlap smoothly. (pic 1)
Using hot glue, attatch the foam rubber strip to the object being used to make the piston. TIP!: The foam rubber can expand when the hot glue is applied, allowing it to be streched out to a greater length slightly. This can be useful when trying to get the perfect diameter for your piston. (pic 2-3)
Use a knife to cut away excess glue from the piston.
Push your piston into the outer cylinder. You may want to use some sort of lubrication to help it slide. After a while, it may loosen up a bit. I use vegetable oil, Crisco shortening, or WD-40 for lubricants, applied with a paper towel to the piston and outer cylinder. (pic 6)
If your pison is made of PVC: You may want to plug the end of your pipe with something, so that whatever will be in the piston does not come out the back of the pipe. For this, I often use my Dremel tool to shape a small piece of plexiglass and glue it into place. A PVC end cap could also be used. (pic 7)

A Little More Theory:
The secret behind a perfectly sealed PVC piston is the foam rubber sheet, mentioned in the previous step. What is great about this material is that it is able to seal like rubber gaskets do, but allows for flexibility in the area of measurements. For example, when you cut ordinary rubber, it has very little, if any, room to adjust its volume. Foam rubber on the other hand, has millions of tiny air bubbles in it, allowing for its volume to adjust when those tiny air bubbles compress.
How does this relate to pistons you ask? When cutting material to make pistons, the material I am cutting is always too big or to small to be able to perfectly seal and move with ease. Foam rubber is able to be cut so that it looks to be the right size, and with a little force, will fit into the outer cylinder and fill it completely. This may be a little hard to understand, the following is a set of steps, describing what happens to the foam rubber as it is forced into the outer cylinder.

The foam rubber is forced into the outer cylinder, compressing the air bubbles within the foam.
Once inside the outer cylinder, the air bubbles within the foam expand, attempting to reach equilibrium with atmospheric pressure.
The air bubbles expansion causes the foam rubber to push against the walls of the outer cylinder, thus creating a perfect seal.

Step 3: The Build: Piston Pictures

Your piston is now completed. I decided to add another step with a couple pictures of the piston put together before going into the Simple Airgun piece of this Instructable. There is also a lot of information of the last page, so this is a good chance for your brain to relax. Pictures of the piston are below.

Step 4: Simple Airgun Theory of Operation

The Simple Airgun is not a lethal weapon, but a fun toy. I wanted something quick and easy to experiment with this method of making a piston. It has only one moving part, and that is the piston itself. So how does it work?

When the volume of a sealed container decreases, the pressure increases, according to Boyle's Law. We hav made a perfectly sealed container with our piston. When we rapidly decrease the volume in the chamber, the pressure will increase, thus becoming able to shoot a projectile.

The gun is composed of two handles. One handle is connected to the outer cylinder, while the other is attached to the piston. The two handles begin apart, and are then pushed together quickly to create an area of high pressure to force out the projectile. See the diagram below.

Step 5: The Build: Simple Airgun

          At this point I already had my perfectly sealed piston, functioning well and well lubricated. I left both ends of the piston unsealed for the purpose of making this airgun.

          I fashioned two gun-looking handles out of a small piece of scrap 1"x6" wood I found in my box of fun things using a combination of the hacksaw and Dremel tool.
          The handles were then glued to the pieces of the piston with hot glue. After a quick and dirty coat of paint, it was done! Check out the pictures!

Step 6: Conclusion

This Instructable reveals the secret to making perfectly sealed PVC pistons for your various desires. The pistons themselves can be scaled up or down and used in many different ways. One such way is in a Simple Airgun, as also described. To fire the gun, simply force the handles together quickly.

Good luck making your own pistons! Comment and rate please!

Thanks for reading!